Table 2.

Regulation principles and material/structure design strategies for PRTM

Functionality	Design principle	Material/structure design strategy
Radiative cooling
IR transparent	Ideally, ${\tau }_{\text{c}}$ = 1 for IR radiation	Chemical bond stretching or bending vibration away from 7 to 14 μm, such as PE, PP, Nylon 6, PTFE, and PVDF. Smaller pore size for Rayleigh scattering of IR radiation
Solar reflective	Ideally, ${\rho }_{\text{c}}$ = 1 for solar radiation	Micro/nanoparticles with high refractive index, such as ceramic or inorganic nanoparticles, like TiO2, SiO2, and Al2O3
Improved IR emissive	Ideally, ${\epsilon }_{\text{c}}$ = 1 for IR radiation	Metamaterials and multilayered nanophotonic structures with high emissivity at ATSW (8–13 μm)
Radiative heating
IR reflective	Ideally, ${\rho }_{\text{c}}$ = 1 for IR radiation	Inclusion of metal-based materials with high IR reflectance, such as metal nanowires, metal nanoparticles, and metal composite, like as AgNWs, Ag NPs, and Cu–Ni NWs
Reduced IR emissive	Ideally, ${\epsilon }_{\text{c}}$ = 0 for IR radiation	Nanophotonic structures coupled with metallic fibers with low IR emissivity, like nanoporous Ag and steel yarns
UV–VIS–NIR and FIR heating	Photothermal conversion for solar radiation absorption	Inclusion of high solar radiation absorptive materials, like CNTs, and dielectric layer, like Ge and ZrC
Dynamic mode
Bilayer emitter	Combination of different emissive materials. High emissive layer faces outside and low emissive layer faces inside for cooling, and otherwise for heating
Bionic materials	Composite materials with self-tunable thermoregulatory properties inspired by the structures of natural creatures, like chameleons and cephalopods
Smart responsive materials	Inclusion of smart self-adaptive fibers that are responsive to environment temperature and humidity to change the yarn width or pore size